The present invention relates to an optical system for an optical pick-up that is capable of using two kinds of optical discs having different recording densities.
In general, diameter of a beam spot formed on a recording layer of an optical disc is closely related to the recording density of the optical disc. That is, the spot diameter is required to have a suitable size for appropriately covering a track on the recording layer in width. The disc with higher recording density has narrower track width, which requires a smaller spot size. On the contrary, the disc with lower recording density has wider track width, which requires a larger spot size.
When the spot diameter is much larger than the track width, reproduced signal may include jitter, which is undesirable effect of the reflected light from adjacent tracks on the reproduced signal. On the other hand, when the spot diameter is much smaller than the track width, particularly for an optical system that reproduces the recorded signal from an optical disc such as a compact disc (CD) through the use of diffraction of light, the system may miss reproducing the signal due to insufficient diffraction.
Since the spot diameter becomes smaller as a wavelength of light becomes shorter and numerical aperture (NA) becomes larger, the optical system for a digital versatile disc (DVD) with high recording density requires shorter wavelength and higher NA, and the optical disc for the CD with low recording density requires longer wavelength and lower NA.
A conventional optical pick-up that is compatible between the CD and the DVD is provided with a pair of optical systems that are specifically designed for the CD and the DVD, respectively. However, such an optical pick-up increases the total size of the device and increases manufacturing cost.
An another conventional optical pick-up is provided with a pair of semiconductor lasers whose emission wavelength are different to each other, an optical system including an objective lens that is common for the CD and the DVD and an aperture mechanism to change NA in response to the type of the disc. However, when the aperture mechanism is mounted on a movable portion that is driven for a tracking servo, the weight of the movable portion increases, which lowers a tracking response.
On the other hand, when the aperture mechanism is mounted on other than the movable portion, and particularly when the aperture limits the beam diameter to be small, the incident light quantity on the objective lens varies due to the tracking movement of the objective lens, which varies a level of the reproducing signal. That is, since the diameter of the incident laser beam into the objective lens is larger than the diameter of the objective lens in general, the tracking movement of the objective lens within the beam diameter does not change the incident light quantity. However, when the Incident laser beam is limited by the aperture, a part of the objective lens may go out of the laser beam due to the tracking movement, which varies the incident light quantity.
It is therefore an object of the present invention to provide an optical system of an optical pick-up, which is a capable of adjusting the beam spot size in accordance with the recording density of the optical disc without using the aperture mechanism.
For the above object, according to the present invention, there is provided an improved optical system of an optical pick-up that is capable of using at least two types of optical discs having different recording density, which includes:
a light source portion for selectively emitting a first laser beam having relatively short wavelength and a second laser beam having relatively long wavelength; and
an objective lens for converging the laser beam from the light source portion onto a recording layer of the optical disc, the objective lens being provided with a transmittance controlling phase grating in a peripheral region to have a wavelength dependence such that the transmittance for the first laser beam is higher than the transmittance for the second laser beam.
The transmittance controlling phase grating is formed as a plurality of concentric grooves. Each of the grooves has a rectangular cross-section in a plane containing an optical axis of the objective lens. Namely, side walls of the grooves are substantially parallel to the optical axis, recessed surfaces of the grooves and protuberant surfaces between the grooves are substantially coincident with the macroscopic shape of the lens surface, respectively. The peripheral region is outside of the transmitting region of the laser beam having a predetermined NA required for the optical disk having lower recording density. It should be noted that the xe2x80x9ctransmittancexe2x80x9d is defined as a ratio of the light quantity of the laser beam that forms a beam spot together with the light beam passing through the central region inside the peripheral region to the light quantity of the laser beam that is incident on the peripheral region.
When the optical disc having higher recording density is used, the light source portion emits the first laser beam having shorter wavelength. Since the transmittance controlling phase grating has higher transmittance for the first laser beam, the first laser beam passes not only the central region but also the peripheral region of the objective lens. As a result, an NA of the objective lens for the first laser beam becomes relatively high, which reduces the size of the beam spot on the optical disc.
When the optical disc having lower recording density is used, the light source portion emits the second laser beam having longer wavelength. Since the transmittance controlling phase grating has lower transmittance for the second laser beam, it cuts off the second laser beam and the second laser beam mainly passes the central region of the objective lens. As a result, the NA of the objective lens for the second laser beam becomes relatively low, which enlarges the size of the beam spot on the optical disc.
In the specification, the objective lens is defined as a lens having at least converging function and transmittance controlling function, irrespective of number of lens element. That is, the objective lens maybe a single lens or a combination of a plurality of elements. Further, the transmittance controlling function may be separated from the converging function. The objective lens may be a combination of a converging lens and a flat plate having the transmittance controlling phase grating.
It is preferable that the transmittance controlling phase grating gives optical path difference that is equal to xe2x80x9cpxe2x80x9d times (p is integer) of the wavelength of the first laser beam and that is equal to xe2x80x9cq+0.5xe2x80x9d a times (q is integer) of the wavelength of the second laser beam. For instance, p=3 and q=2.
Further, the light source portion may be designed such that the first and second laser beams satisfy the following condition (1);
0.81 less than xcex1/xcex2 less than 0.85xe2x80x83xe2x80x83(1)
where
xcex1 is the wavelength of the first laser beam, and
xcex2 is the wavelength of the second laser beam.
Still further, the transmittance controlling phase grating is preferably formed such that a duty ratio is not 1:1. The duty ratio is defined as a ratio of a width R1 of the recessed surface to a width R2 of the adjacent protuberant surface in the radial direction. More specifically, it is preferable that one of the following conditions (2) and (3) is satisfied in at least one pair of the recessed surface and the adjacent protuberant surface;
1.2 less than R1/R2 less than 3.0xe2x80x83xe2x80x83(2)
1.2 less than R2/R1 less than 3.0xe2x80x83xe2x80x83(3)
Preferably, the duty ratio R1/R2 varies depending on a position in a radial direction in the peripheral region. For instance, an absolute difference |R1xe2x88x92R2| may decrease towards the outer side from the inner side in the peripheral region. The transmittance controlling phase grating may satisfy R1 less than R2 in at least one pair of the recessed surface and the adjacent protuberant surface.
The central region inside the peripheral region may be a continuous surface whose cross section along the radial direction is a single curve. In such a case, the light source portion selectively emits the first and second laser beams such that the first laser beam is incident on the objective lens as a parallel beam and the second laser beam is incident on the objective lens as a divergent beam.
On the other hand, a diffractive lens structure may be formed in the central region. The diffractive lens structure is formed as a plurality of concentric rings each of which has a wedge cross section to have wavelength dependence such that spherical aberration varies in the under corrected direction as wavelength of incident light increases. In such a case, the light source portion selectively emits the first and second laser beams such that the first and second laser beams are incident on the objective lens as parallel beams.